Error in building a geant4 program while using cmake

Hello everyone.
I have written a code for simple HPGe detector configuration.
Where I have written codes for

Both header and C++ files. I wrote the main program for this

#include "G4RunManagerFactory.hh"
#include "G4UImanager.hh"

#include "hpgeDetectorConstruction.hh"
#include "hpgePhysicsList00.hh"
#include "hpgeActionInitialization01.hh"

int main()
  // construct the default run manager
  auto runManager = G4RunManagerFactory::CreateRunManager();

  // set mandatory initialization classes
  runManager->SetUserInitialization(new hpgeDetectorConstruction);
  runManager->SetUserInitialization(new hpgePhysicsList);
  runManager->SetUserInitialization(new hpgeActionInitialization);

  // initialize G4 kernel

  // get the pointer to the UI manager and set verbosities
  G4UImanager* UI = G4UImanager::GetUIpointer();
  UI->ApplyCommand("/run/verbose 1");
  UI->ApplyCommand("/event/verbose 1");
  UI->ApplyCommand("/tracking/verbose 1");

  // start a run
  int numberOfEvent = 10;

  // job termination
  delete runManager;
  return 0;

Exactly similar to the code given in the geant4 User Documentation.
Also, I have written the CMakeLists.txt file:

# Setup the project
cmake_minimum_required(VERSION 2.6 FATAL_ERROR)

# Find Geant4 package, activating all available UI and Vis drivers by default
# You can set WITH_GEANT4_UIVIS to OFF via the command line or ccmake/cmake-gui
# to build a batch mode only executable
option(WITH_GEANT4_UIVIS "Build example with Geant4 UI and Vis drivers" ON)
  find_package(Geant4 REQUIRED ui_all vis_all)
  find_package(Geant4 REQUIRED)

# Setup Geant4 include directories and compile definitions
# Setup include directory for this project

# Locate sources and headers for this project
# NB: headers are included so they will show up in IDEs
file(GLOB sources ${PROJECT_SOURCE_DIR}/src/*.cc)
file(GLOB headers ${PROJECT_SOURCE_DIR}/include/*.hh)

# Add the executable, and link it to the Geant4 libraries
add_executable(hpge ${sources} ${headers})
target_link_libraries(hpge ${Geant4_LIBRARIES})

# Copy all scripts to the build directory, i.e. the directory in which we
# build hpge. This is so that we can run the executable directly because it
# relies on these scripts being in the current working directory.

foreach(_script ${HPGE_SCRIPTS})

# For internal Geant4 use - but has no effect if you build this
# example standalone
add_custom_target(hpge DEPENDS hpge)

# Install the executable to 'bin' directory under CMAKE_INSTALL_PREFIX
install(TARGETS hpgeDet DESTINATION bin)

But when I try to build the make file it shows the error like using the command cmake <path>:

-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found EXPAT: /usr/lib/x86_64-linux-gnu/ (found suitable version "2.2.9", minimum required is "2.2.9") 
-- Found XercesC: /usr/lib/x86_64-linux-gnu/ (found suitable version "3.2.2", minimum required is "3.2.2") 
-- Found OpenGL: /usr/lib/x86_64-linux-gnu/   
CMake Error at CMakeLists.txt:66 (add_custom_target):
  add_custom_target cannot create target "hpge" because another target with
  the same name already exists.  The existing target is an executable created
  in source directory "/home/subhrod/G4_project/HPGe_design".  See
  documentation for policy CMP0002 for more details.

CMake Error at CMakeLists.txt:71 (install):
  install TARGETS given target "hpgeDet" which does not exist.

-- Configuring incomplete, errors occurred!

Please guide me towards the fixing of this type of error.